Decision making before micro processors

[harsh2]

i need a circuit to turn a relay on and off.
the relay controls 12 volt fans.

the control condition is heat, which comes from my refrigerator, and the fans vent the heat away from the cooling fins.

so when frige comes on i would like to have the fans come on shortly thereafter.
and when the frige goes off i would like to have the fans go off.

i could even do it without a processor using an a/d converter, but i dont even want to use an a/d converter, seems like overkill.

the circuit should be simple , maybe using an op-amp ,maybe.

options:
thermistor with changing resistance..

but how do i turn a changing resistence into what i need ?
driveing the relay isnt a problem assuming i have a signal to drive it with.

so i have a changing resistance ,and i am working with 12 volts i think the relay is 12 volts also

Thank you.
william

 

[MP1]

Something like this? (See attached)

MP

Temp_Controlled_Fan_w-Thermistor.pdf

 

[harsh2]

thats exactly what i was looking for ;D
i cant read whats written under the circuit
the mic-502 is an op amp?

 

[harsh2]

i found it.. a fan controller huh .. nice...
thanks

 

[MP1]

Here is the data sheet for the MIC502 in case anyone needs it. Also has a nice clear diagram in the data sheet for application.

MP

mic502.pdf

 

[harsh2]

i decided to try a comaprator .. the LM339 quad comparator..
and it worked *yay*.. and is working as we speek.

 

[harsh2]

here is the circuit i built

View attachment 35677

 

[harsh2]

all parts can be gotten at radio shack..

PART LIST

1) Thermistor
1) LM339 Quad Comparitor
1) IFR 510 Mosfet transistor N channel
1) 10K varible resistor
1) 10K resistor
2)330K resistors
1)2.2K resistor

 

[ante1]

harsh,

A suggestion, if you add some positive feedback you get a Schmitt trigger effect and a better on/off switch.

Ante :

 

[ante1]

harsh,

This is what I am talking about: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/schmitt.html

Ante :